Powder feeders are utilized for a wide variety of different applications. One such application is feeding powder into a thermal spray gun which utilizes a combustion flame or arc plasma for melting and propelling the powder to produce a coating on a workpiece. Typical powder feeders for this application are disclosed in U.S. Pat. Nos. 4,561,808 and 4,900,199 of the present assignee.
Such powder feeders comprise a hopper for the powder, with a powder pickup tube passing through or adjacent to the lower part of the hopper. A carrier gas, such as air, nitrogen or other gas compatible with the spray gun or other operation, is forced under pressure through a central passage in the tube. An orifice in the tube wall picks up powder from the hopper for entrainment in the carrier gas and thence conveyance to a thermal spray gun or other point of utilization. A second gas generally is supplied into the hopper to aid in the pickup of powder into the orifice. Control of the pressure or flow of the pickup gas may serve to regulate powder feed rate. The powder pickup tube may have a complex configuration such as a venturi in the vicinity of the powder orifice to aid in drawing in powder.
Some feeders include a mechanical device such as a wheel, screw or disk to feed powder at a controlled rate through an orifice into the carrier gas. Other feeders utilize valves to shut off the flow of powder. For example U.S. Pat. No. 2,707,132 discloses a sliding gate covered with a layer of wear-resistant rubber. The aforementioned U.S. Pat. No. 4,900,199 discloses an elastomer pinch valve closed by application of gas pressure outside the valve, and also discloses an elastomer check valve.
Powders such as those used in thermal spraying can be abrasive to components in a feeding device. Typical of these powders are metals, oxide ceramics and carbides, which generally are in a size range between about 5 and 150 microns. In configurations of feeders where entrainment of the powder by a feed gas into the carrier gas necessarily entails impinging powder against the pickup tube wall, the wall erodes away so as to limit the life of the pickup tube. This can be costly from the standpoints of both replacement parts and down time.